The disclosure herein relates to the field of combustion systems, and more particularly, to a system for reducing emissions in combustion systems.
The reduction of harmful emissions has been a longstanding goal in the design of combustion systems, particularly power plants. The predominant emissions from gas turbine power plants are the oxides of nitrogen, or NOx. The most prevalent NOx emissions are nitric oxide, NO, and nitrogen dioxide, NO2.
Although many combustion systems use natural gas, which is one of the cleanest-burning fuels, the NOx levels of these combustion systems remain relatively high. For example, in the standard household kitchen stove, the burner flame releases NOx emissions at about 48 parts per million (ppm). Other devices such as gas barbecue stands, hot water heaters, and Bunsen Burners also release NOx emissions at approximately that level. Therefore, there is a need to further reduce NOx emissions for combustion systems, particularly in power plants, but also in other combustion systems. Although electricity is the cleanest energy option, NOx emissions still occur, concentrated at the source where electricity is generated (i.e., at the power plants).
NOx emissions are produced by a high-temperature reaction of the nitrogen and oxygen contained in air. Reducing the combustion temperature reduces the level of NOx emissions. However, a reduction of the combustion temperature generally slows down the chemical reaction of carbon combustion, thereby generating high levels of carbon monoxide. For this reason, gas turbine combustion systems and natural gas burning power plants usually use a diluent such as steam or water spray in order to reduce the flame temperature.
Mixing steam and water creates turbulence, effectively increasing the diffusivity of the oxygen to be mixed with the fuel for combustion. Water droplets in the flame front evaporate rapidly, creating a phenomena known as xe2x80x9cmicroexplosions.xe2x80x9d While the injection of steam or water creates turbulence and reduces the flame temperature, the water vapor becomes an additional inert gas (other than nitrogen) with a high heat capacity. It has been shown that the use of such diluents in a gas turbine significantly reduces NOx emission levels, for example, to lower than 25 ppm.
NOx reduction improvements have stagnated, and a need remains to add further reduction means to combustion systems to reduce the NOx emission levels even more. Existing devices can be expensive and difficult to operate, and sometimes even create other emissions themselves. One such device is a selective catalytic reduction system (SCR), which uses ammonia and a catalyst to reduce the NOx emissions. A selective catalytic reduction system can normally reduce NOx emissions by 90% in the flue gas. However, ammonia itself can be a dangerous substance, and under high temperature conditions, ammonia can react violently with water, causing bums and eye injuries. Ammonia also decomposes into nitrogen and hydrogen, which is an undesired and unproductive result. Therefore, there is a need to further reduce NOx emissions of combustion systems through more practical and effective means.
FIG. 1 shows the structure of a typical diffusion flame. The gaseous fuel enters through a nozzle 10 and is supported by a diffusion flame such as a fuel injector or a candle. The flame structure can be simplified into a paralysis zone 12 (shown cross-hatched in the middle), a fuel diffusion zone 14, and a flame surface 16. Oxygen is diffused from the surrounding area toward the flame surface. Under the diffusion flame structure, the combustion reaction can only take place on the flame surface 16 when the fuel and oxidizer reach the stoichiometric ratio. The temperature at the flame surface therefore remains substantially constant and independent of the rate at which fuel is emitted into the nozzle 10. The change to a higher fuel emission rate would cause a larger flame surface.
The heat from the flame surface transfers back to the center of the fuel supply, causing the fuel to be paralyzed into smaller chemical elements such as carbon and hydrogen. These smaller elements diffuse toward the flame surface to support the combustion process. The combustion heat is divided between the combustion products and ambient inert gas. If the surrounding gas is air, then nitrogen will remove some of the heat without participating in the chemical reaction, thereby lowering the overall flame surface temperature. However, if the gas is pure oxygen, the flame surface will reach its highest possible combustion temperature. A gas that does not react with oxygen also can act as an inert gas, removing heat from the flame temperature without participating in the chemical reaction and thereby further lowering the flame temperature.
FIG. 2a illustrates a typical mutual diffusion profile of fuel and oxidizer without combustion. That is, FIG. 2a represents a diffusion phenomena of fuel and oxidizer as a concentration profile with respect to distance from the centerline (i.e., from the source of the fuel or the middle of the paralysis zone) without combustion. The x-axis represents the distance from the source of the fuel. No chemical reaction has taken place in FIG. 2a. In FIG. 2b, when the chemical reaction occurs, in the form of combustion, the concentrations of fuel and oxidizer both approach zero at the flame surface. The concentration of the combustion products is highest at the flame surface. Despite the disappearance of fuel and oxidizer, however, the flame maintains the diffusion rate present when the concentrations of fuel and oxidizer are at the stoichiometric ratio, as illustrated in FIG. 2a. 
FIG. 3 illustrates the flame height as a function of turbulence level with an increasing fuel nozzle jet velocity. The left side shows a very long flame having a height that increases along with the fuel jet velocity. The flame is a laminar flame. The right side shows the flame as the fuel jet velocity increases. Although the height of the flame decreases at first, an increase of the fuel jet velocity eventually keeps the turbulent diffusion flame at a constant height. With the laminar flame on the left side, the flame diffusion is strictly molecular. Therefore, the surface area of the flame remains proportional to the fuel ejection rate from the fuel nozzle. When the velocity continues to increase, it induces turbulent mixing which greatly increases the molecular diffusivity. The jet of the fuel nozzle finally reaches a condition known as a similarity flow, which means that the flame is at a constant flame height. The similarity flow occurs when the turbulent mixing profile becomes independent of the magnitude of the velocity.
When the chemical reaction rate is slower than the turbulent diffusion rate, the flame will be lifted from the fuel nozzle, creating a blowout condition. FIG. 4 illustrates combustion flame profiles with respect to blowout conditions. FIG. 4a illustrates the condition of fuel with an extremely high jet velocity. In order to improve the chemical reaction rates and stabilize the flame, some of the combustion products are recirculated through turbulent mixing as chemical reaction seed material. The bell-shaped profile in FIG. 4a illustrates the root of the flame, and the cone-shaped region represents the turbulent combustion of fuel and air. When the velocities of both fuel and air increase, the root of the flame lifts away from the nozzle, leaving certain recirculation chemical species to support the combustion. As illustrated in FIG. 4b, when the velocity increases, the recirculation is reduced, causing the flame to lift away from the nozzle and creating a neutral condition. FIG. 4c illustrates the results of a maximum increase in the velocities of both the jet and air. Chemical species can no longer recirculate, and the flame completely lifts from the nozzle, creating a blowout condition. Candles illustrate this phenomena well: when one blows gently on a candle, the combustion rate of the candle increases. However, as one blows harder on the candle, the combustion rate catches up to the diffusion rate, thereby extinguishing the flame.
As mentioned above, a particular need is to reduce the level of NOx emissions in gas turbines. The publication xe2x80x9cFundamentals of Gas Turbines, Second Edition,xe2x80x9d William W. Bathie, provides a detailed description of gas turbines, and is hereby incorporated by reference. FIG. 5 illustrates a typical gas turbine combustion system. The outside liner 20 has many dilution holes 30. A pre-mixing swirler 40 surrounds a fuel nozzle 50. The dilution holes 30 create a recirculation flow which serves to guide the combustion product back into the primary combustion zone to help accelerate the chemical reaction of combustion. The swirler 40 creates the fundamental turbulent mixing for the fuel jet as the fuel exits the hole 51. This design uses recirculation and turbulence to establish a similarity flow. The combustion products then mix with dilution air through the dilution holes 30 to reach a final temperature before entering the nozzle of the gas turbine.
FIG. 6 illustrates prior art devices used in the industry. A concentric nozzle 61 has fuel and diluent injections for creating a turbulent flame. Specifically, one conduit supplies fuel, while the other supplies steam or water. The concentric nozzle 61 is surrounded by another system 63. The turbulence of the fuel, and the high velocity of the diluent (such as a steam jet or water spray), usually create the flame mixing region. The steam, fuel, and air are mixed while burning or combusting. A problem with this prior art device is that the length of the mixing depends on the geometry of the nozzle for a turbulent jet; therefore, the concentrations are not homogeneous. Some places have more fuel than other places, which does not ensure that the steam, fuel, and air is a homogenous mixture. As a result, xe2x80x9chot spotsxe2x80x9d are produced and the NOx level is relatively high. As explained with regard to FIG. 1, the temperature of the flame surface is uncontrollable by bulk mixing. In fuel-rich regions, the flame temperature can still reach a very high level and produce NOx.
xe2x80x9cHomogenousxe2x80x9d as used in this specification means a concentration deviation from the average, with average being 100% homogeneous. For example, if a closed vessel contains on average 50% fuel and 50% air, and in a localized region actually contains 49% fuel and 51% air, then the concentration deviation from the average, or from the overall ratio of components, is 2%, denoting 98% homogeneity.
The concentration deviation from the average of prior art devices using turbulent mixing is believed to be in the approximate range of 15%-25%, or, a range of homogeneity from 75%-85%. It is an object of the disclosure herein to significantly improve upon the percentage of homogeneity present in prior art combustion systems.
FIG. 7 illustrates a traditional coaxial mixing of a jet of fuel surrounded by another gas (in this case, air). The solid contour lines represent fuel concentration. For example, a fuel concentration of 0.1 represents 10% fuel and 90% air. Although 1.0 is not marked on the figure, it is indicated by the last contour of fuel coming out over the nozzle. The data relating to FIG. 7 showed that even at more than 20 diameters downstream of the fuel nozzle, the homogeneous mixing was nowhere near completion. Therefore, the turbulent flame creates uncertainties in terms of concentration fluctuations as represented by the dash lines in the region containing a 50/50 mixture average. If the surrounding gas is steam, then this mixture represents rich and lean regions of fuel mixed with steam. The turbulent properties and fluctuation intensity of this mixture subject it to different temperature fluctuations. Unfortunately, a region with a higher fuel concentration will have a higher flame temperature, and, consequently, produce a higher level of NOx emissions. Because of this, this prior art nozzle design has not been able to achieve a NOx level below approximately 20 ppm in gas turbines burning natural gas.
FIG. 8 shows typical plots of NOx and CO productions based on a well-stirred combustion situation as a function of flame temperature. This graph was generated assuming that the turbulence levels were high enough for combustion to occur at ratios other than the stoichiometric ratio. These plots illustrate the best attempts at reducing NOx productions with a highly turbulent, lean, well-stirred combustion situation. Previously used as the most advanced technology in gas turbines, these systems are called Dry Low NOx Combustion Systems (DLN). The word Dry (D) indicates a lack of mixture with steam or water. It is clear that further NOx reductions are needed.
One object of the disclosure herein is to reduce the level of NOx emissions in combustion systems well below that of natural flame processes. To achieve this object, the disclosure herein teaches to homogeneously pre-mix the fuel with a diluent, such as steam, before it enters the diffusion flame system. To eliminate the hot spots in a turbulent flame, the concentration distribution of a turbulent jet using the teachings of the disclosure herein becomes uniform. Another object of the disclosure herein is to simplify combustion systems by using a static mixer to save space in the system. Another object is to sustain lean combustion without flameouts, using homogeneous mixing and a pilot third gas. Ultimately, the disclosure herein greatly reduces NOx emissions in combustion systems at a decreased cost by means of a simplified arrangement.
The disclosure herein in a preferred embodiment provides a method for reducing emissions in a combustion system, comprising the steps of creating a mixture of diluent and fuel, wherein the diluent and the fuel are at a predetermined diluent-to-fuel ratio, homogenizing the mixture to create a homogenized mixture having a uniform concentration distribution of the diluent and the fuel at the predetermined diluent-fuel ratio, and, thereafter, introducing the homogenized mixture into a flame zone and combusting the homogenized mixture.
The diluent can be steam. The homogenizing step can be performed by a compact mixer. The homogeneity of the homogenized mixture is preferably in the range of 97-99%. A third gas such as air, hydrogen, or hydrogen peroxide may be added to the mixture before the homogenizing step. The predetermined diluent-to-fuel ratio is preferably in the range of 0.2 to 1, or 0.2 to 3. xe2x80x9cRatioxe2x80x9d as used in this specification means the ratio by weight of components.
The disclosure herein in another embodiment provides a gas turbine. The gas turbine has a compressor and a chamber disposed downstream of the compressor for receiving diluent and fuel at a predetermined diluent-to-fuel ratio to form a mixture. A compact mixer is disposed downstream of the chamber for homogenizing the mixture to create a homogenized mixture having a uniform concentration distribution of the diluent and the fuel at the predetermined diluent-fuel ratio. A combustion section is disposed downstream of the compact mixer for combusting the homogenized mixture after the homogenized mixture leaves the compact mixer to produce a hot energetic flow of gas. A turbine is disposed downstream of the combustion section driven by the hot energetic flow of gas for driving the compressor.
Experiments have proven the teachings of the disclosure herein, wherein the mixture of gaseous fuel and diluent is homogenized, to be effective for reducing emissions in combustion systems.